Rotating shafts, such as those employed in motors, engines, compressors, generators, and the like, are often subjected to axial thrust loads. The thrust loads are axial in that they are directed parallel to the longitudinal axis of the rotating shaft.
Thrust bearings have therefore typically been employed to address such thrust loads and maintain proper axial positioning of the shaft. Thrust bearings are available in different forms including electromagnetic and conventional mechanical thrust bearings. With an electromagnetic thrust bearing a metal rotor is attached to the shaft subjected to the thrust loads. One or more stators, depending on the number of potential directions in which the axial thrust loads may be directed, are then disposed adjacent the rotor to generate magnetic flux. The magnetic flux acts against the metal rotor attached to the shaft and counteracts against the axial thrust loads to thereby maintain proper axial positioning of the shaft. U.S. Pat. Nos. 5,101,130 and 5,315,197 are indicative of typical electromagnetic thrust bearings of this type.
Jayawant et al '130 discloses an axial magnetic bearing suitable for use where high loads and high rotational speeds are found together. The bearings comprise an assembly where, for thrusts along the shaft in one direction, the shaft has in relation thereto, two axially spaced, and radially extending, thrust accepting faces, a generally channel-shaped, annular electromagnet surrounding the shaft, with a radially extending face of each pole of the electromagnet adjacent to an individually associated thrust face.
Meeks et al '197 discloses an electromagnetic thrust bearing which couples a rotatable member relative to a stationary member utilizing a combination of controllable electromagnets and a radially polarized permanent magnet, each physically associated with the stationary member. In one embodiment, the rotatable member comprises a shaft having a pair of axially spaced apart thrust discs fixed thereto. A pair of solenoids are disposed about the rotatable member between the spaced apart thrust discs on oppositely facing sides of the thrust discs. The solenoids are capable of generating a controllable electromagnetic field. An arcuate, radially polarized, permanent magnet is disposed between the solenoids to generate a constant flux, high density magnetic field between a solenoid housing and the thrust discs. In a second embodiment, a pair of solenoids are disposed about the rotatable member on opposite sides of a single thrust disc. A radially polarized permanent magnet is disposed between the solenoids radially outwardly from the thrust disc for generating a constant flux high density magnetic field between solenoid housings and the thrust disc. A sensor determines the axial positioning of the rotatable member relative to the stationary member and provides input for controlling the positioning of the thrust discs by varying the magnetic flux generated by the solenoids.
At high rotational shaft speeds, an electromagnetic bearing rotor may experience structural fatigue and failure. It would therefore be desirable to reduce the diameter of the bearing rotor in such high speed applications. However, the degree to which the diameter can be lessened is, in part, limited by axial thrust loads to which the shaft is subjected. If the axial thrust load is relatively high, the flux path area and the magnetic flux generated by the bearing will necessarily need to be relatively high. This in turn requires a larger electromagnetic coil. Since the poles of the electromagnetic bearing are ideally located at the innermost and outermost radii of the rotor, the larger electromagnetic coil requires a higher diameter rotor, which as indicated above is prone to structural fatigue and failure at high rotational speeds.
Systems have therefore been developed which provide a sufficiently sized electromagnetic coil to counterbalance axial thrust loads, but which also decrease the physical gap between the stator poles, and thereby allow for a decreased diameter rotor. For example, U.S. Pat. No. 5,406,157 discloses an electromagnetic bearing arrangement wherein the stator includes radially inner and outer segments with a groove therebetween for receipt of the electromagnetic coil. The open ends of the radially inner and outer segments define the poles of the electromagnetic bearing, with the open end of the radially outer segment being tapered radially inward to decrease the distance between the poles and thereby allow for a decrease in the outer diameter of the rotor.
While such a system does allow for a reduced diameter rotor, a need exists for a more structurally rigid stator which is able to withstand greater loads and "oil-canning". In addition, a need exists for an electromagnetic coil which maximizes the density of the individual turns of the electromagnetic coil within the stator, and thereby maximizes the electromagnetic flux generated by the bearing. Moreover, a need exists for an electromagnetic bearing which maximizes the electromagnetic flux density of the bearing directly at the inner and outer peripheries of the rotor attached to the shaft. Furthermore, such needs must be addressed while decreasing the distance between the stator poles to thereby decrease windage losses and increase the efficiency of the bearing.